This invention relates to semiconductor devices, and more particularly to laser programming of such devices.
Various methods have been employed for selectively programming semiconductor devices after manufacture is essentially complete. For example, in the manufacture of very high density dynamic RAMs, such as the 1-Megabit device, it has become an economic necessity to utilize redundancy; the DRAM devices are programmed after testing in order to bypass faulty parts of the memory. In one commonly used method, a polysilicon or other conductive strip on the surface of the chip is melted through by a laser beam at the desired positions; this is known as laser break-link programming. In bipolar PROMs, conductors are melted at narrow high resistance areas by electrical pulses. In some devices of this type an oxide is broken down by a laser beam or by over-voltage, shorting together two conductors and providing make-link instead of break-link programming. The heating effect of a laser beam has also been used to diffuse impurity into a silicon or polysilicon area to change its conductivity providing a programming method. The effect of laser heating for altering the dopant distribution in silicon devices has been reported.
Prior devices of this type have required excess space on the chip for the link stucture, or for the circuits needed to program the links. The laser-blown fuses have created problems due to the craters produced in the surface of the chip by the laser beam, or splashing of metal or conductive material to other parts of the circuit.
It is the principal object of this invention to provide an improved method of programming of semiconductor devices, particularly by laser beam make-link programmable elements. Another object is to provide a laser programming method which requires less space on the semiconductor substrate for implementation. A further object is to provide a laser programming method which is less disruptive of the surrounding structure and materials, and/or which leaves a minimum of residue. Other objects include lower dwell time needed for the laser beam (thus faster programming) and lower power (thus less heating).